Schizostylis Kafir Lily - One of my favourites and never failing to put on a disply in late summer - or even through to the winter. Evergreen grass-like foliage and rhizome roots below. Easy to divide.
Scilla Flowering Bulb - Low growing bulbous perennial for front of border, rockery or patio tub. Two or three distinct types, but blue being the best colour, and in particular the Scilla siberica with its drooping bell flowers. They need to be kept dry in the summer dormancy period - as with many other 'alpine' type bulbs. Prolonged wet summer months will cause deterioration.
Stipa - Ornamental Grass - Unless you are set against ornamental grasses - and some are - there is bound to be a Stipa which is suited to your garden. Wide range of grasses which includes the graceful Giant Golden Oats. There are also some smaller mound forming types with good foliage and flowers.
Sagittaria - Arrowhead refers to the shape of the individual leaves - pointed and sculpted just like an arrow hear. A marginal perennial for pond use.
Salvia - This group of plants - which do not include the annual tyeps - are hardy to varying degrees. Many are fully hardy. The group includes the 'culinary' sages which are also ornamental by way of foliage, and also several group of erect or clump forming herbaceous perennials - most with the useful feature of being late summer or autumn flowering. Generally the flowers are held aloft on narrow spikes. many new varietioes available, and well worth planting.
Sanguinaria - Bloodroot - I found one peeping up through leaf littler in the dead of winter, and have been a fan ever since. Low growing rhizome rooted perennial that flowers as the emerging folded foliage unfurls. White cupped star-like flowers. Prefers a woodland situation.
Sanguisorba - Burnet - Bottlebrush flowers held erect and normally white. Height ranges from 60cm to around 1.5 dependent upon variety. Prefers a moist or even damp area in full sun or dappled shade.
Saponaria - Soapwort. I had many visitors to my front door politely enquiring what the name of 'that' plant is! It being a Saponaria ocymoides draped down a low retaining wall in afternoon sun. Generally evergreen foliage, and profusion of pink flowers.
Saxifraga - Saxifrage - Mossy Saxifrage or rosetted leaves - there are also other forms. A wide group of evergreen low growing perennials that generaly prefer light shade - though some will grow in quite heavy shaded areas. Flowers normally held upright on slender stems, but some arch over to touch ground. A good group.
Scabiosa - The Pin Cushion Plant. Predominantly light blue flowers but with some pink and white variations. A good choice for an open area and alkaline soil, but will also grow in normal 'neutral' soil types. Hate being in wet soil through the winter, but otherwise hardy.
Scoliopus - Bi-foliaged and twin petals on this unusual rather than beautiful slightly spreading herbaceous perennial. Not easy to obtain, but worth it for its 'novelty' value if you can get one.
Scrophularia - Figwort - One for the bog garden or very shallow marginal.
Scuttelaria - Helmet Flower - Low growing clump forming herbaceous plants - generally for rockery or at front of shrub - perennial border.
Sedum - Every year sees new introductions to this valuable group of summer and late flowering, sometimes evergreen perennials. there are minute jobs for the rockery and the larger Ice Plant types for the borders. All are happy in full sun, with the larger leaved types also suited to dappled shade.
Selinum - One of the Carrot family, that has finely divided ferny foliage, and flattish plates of foam like flowers. Imposing perennial that can grow to 2m high. Really different if you have room, though it takes up less room than its height suggests.
Sempervivum - Love them or hate them! houseleeks have a lot going for them, including the fact that they are prepared to grow where many other plants cannot. Low growing, evergreen, no maintenance - oh and dainty flowers as well. case proven. Grow some!
Senecio - Many to choose from, including the silver leaved Senecio cineraria 'Silver Dust' which is very hardy if planted right! Main requirement is a well drained full light situation.
Seriphidium - I do not know it, but looks suspiciously like an artemesia to me. Will investigate.
Seslaria - Ornamental Grass - Evergreen clump of sometimes silver foliage on the ornamental grass, with interesting cylinders of white flowers.
Sidalcea - Pink - almost shocking spikes of mallow-type flowers - hence the name of Sidalcea malviflora! Will grow anywhere, any soil, but not in dense shade. But, it will brighten up any dark corner of the garden.
Silene - Campion - is generally low growing with masses of small flowers - sometimes clusters. All shades of pink and a few whites thrown in.
Silphium - Perfect if you want a perennial that has height but does not intrude too much at ground level. S. laciniatum has finely cut and divided leaves as a foil for the yellow daisy flower held high.
Sisyrinchium - low growing for the rockery or trough, but also some reasonable size semi-evergreen. Foliage similar to Iris, including the variegated one. Flowers mainly cream on the taller (60cm) withn blue being the predominant colour of the small Alpine types. Easy to grow - need a dry spot, and just love full sun.
Smilacina - Spreading herbaceous perennial similar to Solomon's Seal, and therefore best in a woodland setting. Happy in shade - even dense shade.
Smyrnium - Very attractive short lived perennial for woodland garden or semi shaded area. Best treated as a biennial, but the lime green flowers and foliage make this a plant well worth having. I only know this as a low spreading plant, but the RHS give its height at up to 1.5m. (Disagree! - I think they meant the 'spread' can be 1.5 metres.) Take it from me, it gets to maximum 60cm, and I do mean maximum.
Soldanella - A fully hardy perennial for the rock garden but please give it some drainage and protection for the winter. Round heart shaped leave with dropping bell flowers - but frilly. Lovely plant.
Solidago - Golden Rod, spreads everywhere! Upright stems of golden clusters - racemes - of flowers give it its name. Will grow anywhere, and is often seen as a weed.
X Solidaster - Clusters of Chrysanth-like flowers on upright growing perennial. Flower in mid-summer, but is normally faded by Autumn. However a good bright addition in the 'yellow' section.
Sphaeralcea - S. munroana is the choice for a hardy perennial - others are sub-shrubs. Bright orange dishes of flowers on erect stems to around 60cm. you will need to protect in winter and pre-empt any problems by planting in a dry situation.
Stachys - Lamb's Ears are typical of the silver foliaged types, but there are others which are grown for their flowers. Some not too fussy as to planting position, but the low growing types are more demanding of a dry well drained situation.
Sternbergia - Flowering Bulb - Often mistaken for a crocus, this bulbous perennial flowers in the Autumn. It is not the 'Autumn Crocus' and in fact has the common name of Autumn Daffodil. A Narcissus it is not, but a member of the Amaryllis family.
Stylophorum - Herbaceous perennial not unlike a golden dwarf poppy. here you have the Celandine Poppy, growing to around 20 - 40cm tall.
Symphyandra - Bell-shaped flowers of the Campanula family adorn the stems of this long flowering herbaceous perennial. To all intents and purposes it is a Campanula - sharing all the main traits of the massive family of ornamental perennials. Some are monocarpic, which is to say that they live for two years but flower and fruit only the once and then die.
Symphytum - Comfrey. There are some useful members of this group of truly herbaceous perennials. they all do well in shaded or dapple shaded area, and the variegated foliage types certainly keep their colours best when not in full sun. As well as the mainly erect habits, there is a very good shade loving ground cover plant by way of Symphytum tuberosum.
Synthyris - Interesting group of mainly low growing,
clump forming herbaceous perennials, with tightly\y knit clusters of
flower spikelets which are of of the blue shades. All have interesting -
but varied - foliage. Suit front of border for none of them grow more
than 10in 25cm tall.